//  Copyright (c) 2010, David Van Puyvelde, Sales Engineering, Salesforce.com Inc.
//  All rights reserved.
//
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trigger PreventTemplateOrphans on PageTemplate__c (before delete) {

	//get the templates that have pages attached (via a count query on pages)
	List<AggregateResult> templateswithchildren = [SELECT PageTemplate__c, count(Id) FROM Page__c where PageTemplate__c in:Trigger.oldMap.keySet() GROUP BY PageTemplate__c ];
	
	//drop them in a Set 
	Set<ID> parents = new Set<Id>();
	for(AggregateResult ar:templateswithchildren) {
		parents.add((ID)ar.get('PageTemplate__c'));
	}
	//check if any of the pagetemplates belong to the 'parents'
	for(PageTemplate__c pt:Trigger.old) {
		if(parents.contains(pt.Id)) pt.addError('You can not delete a template as long as there are pages that use it.');
	}

}